Holding cladding for laser slabs

ABSTRACT

The invention disclosed in this application provides a unique cladding structure for holding laser disc elements structurally rigidly aligned and allows for release of stress produced by heating of the disc element. One unique material for this is &#39;&#39;&#39;&#39;FEP&#39;&#39;&#39;&#39; Teflon which is a Copolymer of Hexafluoropropylene and Tetrafluoroethylene in conjunction with ruby.

United States Patent [191 Nicolai et al.

[ Oct. 16, 1973 HOLDING CLADDING FOR LASER SLABS [75] Inventors: Van 0.Nicolai, Reston, Va.; Harry W. Fox, Washington, DC.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

22 Filed: Sept. 26, 1972 21 Appl. No.: 292,386

[52] US. Cl. 331/945, 330/43 [51] Int. Cl. Hols 3/02 [58] Field ofSearch 331/945; 330/43 [56] References Cited UNITED STATES PATENTS3,621,456 ll/1971 Young ..331/94.5

3,628,172 12/1971 Matovich et a1 331/94.5 3,702,976 11/1972 Young331/94.5 3,711,785 1/1973 Zitkus 331/94.5

Primary Examiner-William L. Sikes Attorney-R. S. Sciascia et al.

[5 7] ABSTRACT The invention disclosed in this application provides aunique cladding structure for holding laser disc elements structurallyrigidly aligned and allows for release of stress produced by heating ofthe disc element. One unique material for this is PEP Teflon which is aCopolymer of Hexafluoropropylene and Tetrafluoroethylene in conjunctionwith ruby.

8 Claims, 7 Drawing Figures HOLDING CLADDING FOR LASER SLABS There hasbeen defined a need for a laser delivering a l kilojoule per pulse,milliradian, 1 percent efficiency, and with pulse lengths suitable bothfor range resolution and for long range imaging. A major limitation onthe design of a kilojoule system is the pump intensity. It has beenshown that above a pressure of 0.1 atmosphere the electron collisionrate becomes so large that the electron temperature and the gastemperature become very nearly equal. As temperatures are increased bygoing higher currents and pressures, any spectral lines tend to broadenand disappear into the rising continuum, and the spectral distributionapproaches a black body distribution. Thus a flash lamp with a spectraldistribution close to that of a black body will give a higher rubypumping rate than spectrally matched lamps. Black body temperatureshigher than 9,000 K give only slightly increased ruby pumping and a lotmore UV, resulting in less overall efficiency.

In order for a kilojoule pulse to be easily directed at a distant targetit should be contained in a single coherent beam. A loss coefficient inactive ruby of about 35 percent per meter prevents the use of amplifierslonger than about a meter; maximum ruby diameter currently available isabout 4 inches. The resulting limit in cylindrical amplifier pumpingarea and the limitation in lamp intensity make pulse lengths less than100 microseconds practical only for the storage mode of operation. Withan artificial delay line matched to the lamp impedance the energy can bepumped in uniformly during a l millisecond pump time, stored, and thenremoved in either 100 nanoseconds for both range imaging and rangeresolution or removed in microseconds if only long range imaging withless chance of ruby damage is desired.

Low beam divergence requires that the ruby be uniformly pumped. Uniformpumping is achieved only when ruby doping densities are small enough totransmit 90 percent of the pump light through the ruby in a single pass.This 90 percent is reabsorbed by the lamps. Completely surrounding theruby by flash lamps will give maximum pumping intensity but so muchflash lamp wall area, through which heat is conducted, results in aninefficient system. With spaced lamps and interspaced reflectors thelamp wall area is reduced and efficiency is higher with only a smalldecrease in overall pumping intensity due to absorption by thereflector. An odd number of lamps places a reflector section opposite adirect emitting lamp section. Thus, an odd number of lamps should givebetter uniformity of pumping with respect to azimuth. Five lamps seemsto be an optimum number for the larger diameter rubies. For a smalldiameter oscillator three lamps gives better proportions, etc.

Several workers have published absorption cross sections for the excitedstate of ruby as a function of pump wavelength. At pump-band wavelengthsthe absorption cross sections of the excited state is roughly one halfof the ground state absorption. At 0.25 microns the excited stateabsorption is two orders of magnitude greater than the pump bands. Thisexcited state absorption convert pump light into heat when the chromiumatoms relax back down to the R excited state. Since this releaxationtakes place in 1011 seconds, this absorption does not affect the Rpopulation, but it does deposit detrimental heat into the ruby. The 0.25micron absorption is particularly detrimental since it will all beabsorbed at the ruby surface and produce thermal stresses. Wavelengthsshorter than 0.21 microns produce free charge carriers leading to theabsorption of laser light, plasma formation and destruction of the ruby.For these reasons wavelengths shorter than 0.38

microns should be filtered out before they get to the ruby.

A ruby cooling fluid used in this disclosure is perfluorohexane, whichboils at 57 C. All saturated perfluorocarbons transmit from 4 micronsout into the ultraviolet and will therefore not be decomposed by thefiltered pump light. All pump light from 0.38 microns to 4 microns notabsorbed by the ruby, the UV filters, or the reflectors will eventuallyfind its way back to the lamps and be reabsorbed. Since n 1.4 forsaturated fluorocarbons, n 1.768 for ruby and n 1.925 for yttralox,there should be reasonably good optical coupling between the ruby andthe lamps.

Cooling of the Brewster angled ruby slices requires that they be heldfirmly but allowed to expand thermally without developing stresses.Since a glass cladding does not absorb heat likeruby does, thermalstresses will be present in that holding technique. A transparentplastic material FEP Teflon, a Trademarked product of DuPont Companywhich is a C0,- polymer of l-lexafluoropropylene and Tetrafluoroethylenehas physical and chemical properties similar to teflon and offers thebest choice for a cladding material. There are techniques for coatingmaterials with teflon at sintering temperatures around 700 F. Since thechemistry is similar, such techniques would be applicable to claddingruby slabs with FEP Teflon.

It is therefore an object of this invention to provide an improved laserdisc element.

It is yet a further object of this invention to provide an improvedlaser disc element having an unique design for stacking along withsimilar elements to provide-an unique, simplified liquid coolant flow.

Still a further object of this invention is to provide an improved laserdisc element having a cladding material transparent at laser pumpfrequency, and lasing frequencies to provide stress relief for thermalstresses generated in the lasing elements.

Yet a further object of this invention is to provide an improved laserdisc element having a unique plastic cladding for a laser disc elementdesigned to provide uniform flow across the disc faces and seriescoolant flow along a plurality of disc faces to provide a minimumthermal stress because of coolant temperatures.

And still a further object of this invention is to provide an improvedlaser disc element having a fluorocarbon plastic cladding for a rubylaser disc element that provides plastic flow in the fluorocarbonplastic due to thickness stresses of the laser disc element.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of theinventionwhen considered in conjunction with the accompanying drawingwherein:

FIG. 1 is an end view of one embodiment of the invention.

FIG. 2 is a section view along lines AA in FIG. 1. FIG. 3 is a sectionview along lines BB in FIG. 1. FIG. 4 is an end view of fluorocarbonplastic clad ruby laser disc element. FIG. 5 is a section view alonglines CC in FIG. 4. FIG. 6 is a side view of a second embodiment of theinvention.

FIG. 7 is an end view of the laser disc element in FIG. 6.

The FIGS. 1, 2, 3, 4, and taken in conjunction show a first embodimentof an assembly of lasable elements mounted along a longitudinal axis 10and more specifically they consist of the following.

Individual lasable disc elements 11, 12, 13 and 14 are shown positionedalong axis 10 at a pre-determined angle represented by 15. Dependingupon the composition of the lasable elements and the liquid coolanthereinafter described, this angle can vary from zero to the Brewsterangle for the lasable material.

FIGS. 4 and 5 show the specific construction of the lasable element 11which is substantially identical to all other disc elements 12, 13 and14. Surrounding the element 11 is a cladding material 16 which in thiscase is fluorocarbon plastic and on the other elements as shown 17, 18,and 19 which has a unique configuration.

Lasable element 11 has a thickness designated as 20 and a radius 21. Inthe embodiment shown it should be noted that faces 22 and 23 of the discelements are substantially at the Brewster angle for the material, inthis case ruby.

The cladding material 16 surrounding the entire disc material at athickness 20 and has upper and lower extensions 24 and 25 which providesspacing between disc elements when they are stacked along axis 10.Additionally there is shown in FIG. 4, a portion of the claddingmaterial removed, or never casted, which is the area shown between thedash lines and cladding outer circumference 31 which is designated as avoid or recess 32. When the disc element is assembled in the totalassembly as shown in FIG. 3, portion 32 leaves a void in conjunctionwith a cylindrical holding element 34 to allow for the passage ofcoolant in a predetermined manner.

Similar recessed portions of the cladding material 36, 37 and 38 areprovided on disc elements 12, 13 and 14. Somewhat schematically shown isan inlet port 40 and an outlet port 41 which are provided for flow ofcoolant liquid shown filling all of the space between the disc elements.

Block 43 represents a pump which is coupled via line 44 to the exit portwhich represents a means for return flow. Of course it should be notedthat cooling of the coolant liquid 42 would be provided in such a systembut a detailed description is not necessary for an understanding of thisinvention.

Arrows 50, 51, 52, 53, and 54 show the direction of liquid flow which isprovided by the unique construction of the disc and cladding elements.The discs are stacked along axis 10 so that the recessed portion orvoids 32, 36, 37, and 38 are 180 or on alternate sides. This uniquearrangement allows for a series type of coolant flow past the discelements and depending upon design requirements provides a maximum ofcoolant and a minimum of temperature gradient across the disc due todifferences in coolant temperatures. In other words the coolant adjacent14 is warmer than the coolant adjacent l1 and the temperature gradientbetween liquid flowing represented by arrows 53 and 54 adjacent to facesof disc 14 has a minimum thermal differential and therefore the coolantintroduces a minimum of thermal stress. If the same amount of heatingwere to be removed in a parallel arrangement then the coolant might wellintroduce greater thermal stress than are allowed.

FIG. 5 shows the configuration of the recessed portion which is designedto provide a minimum turbulence and at the same time accomplish thedesired results. This is accomplished by having the distance along thecircumference of the cladding material 16 represented by line 31determined by a chord from point 60 and 61 represented by line 62.

This line is equal to the diameter of a face of the disc 1 1.

In FIG. 1, flash lamps and 71 are shown to schematically position to thestack of lasable elements within a system. It should be noted that FIG.6 shows a second embodiment of the invention having a lasable discelement 80, a cladding element 81 and an outer transparent holdingelement 82. Such construction is desired if it is desired to provide aplurality of stacking elements which would incorporate housing element34 in sections.

FIG. 7 which is an end view of such construction, shows the removedportion 83 which provides a passage for the liquid. Dash line 84represents an alternate construction wherein the liquid flow would bethru the outer portion of element 82. In this construction, the outerelement 82, could be a sturdy element such as glass and element 81, aplastic element such as a solid perfluorocarbon allows thermal expansionby the laser element to be absorbed in the plastic element by plasticflow and thermal stress in element is relieved.

A construction such as 84 would mount the lasable element 80 in thecenter and allow uniform plastic flow of cladding 81 which would tend tokeep the disc coolant aligned to a higher degree. Obviously manymodifications and variations of the present invention are possible inthe light of the above teachings. It is therefore to be understood thatwithin the scope of the appended claims the invention may be practicedotherwise than as specifically described.

I claim:

1. An assembly of lasable elements alignable along an axis comprising;

a. a plurality of cylindrical lasable elements;

b. a plastic cladding material consisting of a transparent plastic abouteach of said elements;

c. a means about each of said cladded elements for holding said elementand for aligning said lasable element along said axis at apre-determined angle, said plastic material having properties wherebyheating of the lasable element due to pumping and lasing which causessaid lasable element to have thermal stresses therein allows saidelement to expand causing plastic flow of said plastic cladding andreduces the thermal stress within the lasable element.

2. The device of claim 1 wherein said plastic cladding material is aCopolymer of Hexafluoropropylene and Tetrafluoroethylene.

3. The device of claim 2 wherein said lasable elements are substantiallydisc shaped having a diameter substantially larger than the thickness ofsaid disc.

4. The device of claim 3 wherein portions of said cladding material havea length greater than said thickness of said lasable element to providespacing between said elements when aligned in said assembly along saidaxis for allowing the flow of a cooling liquid past the faces of saidelements.

5. The device of claim 4 wherein a portion of said cladding material isremoved about the outer circumof two substantially equal parts atopposite sides of said element and defined therebetween a channel toallow cooling liquid to flow past the entire face of said disc elements.

8. The device of claim 7 wherein alternate ones of said lasable elementshave cladding removed from a portion of the circumference of saidcladding and the removed portion is substantially from adjacent elementsso that coolant liquid flow is in a series arrangement past faces ofsaid lasable elements.

2. The device of claim 1 wherein said plastic cladding material is aCopolymer of Hexafluoropropylene and Tetrafluoroethylene.
 3. The deviceof claim 2 wherein said lasable elements are substantially disc shapedhaving a diameter substantially larger than the thickness of said disc.4. The device of claim 3 wherein portions of said cladding material havea length greater than said thickness of said lasable element to providespacing between said elements when aligned in said assembly along saidaxis for allowing the flow of a cooling liquid past the faces of saidelements.
 5. The device of claim 4 wherein a portion of said claddingmaterial is removed about the outer circumference of said claddingmaterial to allow for passage of cooling fluid past one face of thelasable element to the other face of said lasable element to provide aseries type flow of said cooling liquid past the individual lasableelements.
 6. The device of claim 5 wherein the protion of removedcircumference of said cladding material is at least a chord subtended byan angle equal to the diameter of said disc element.
 7. The device ofclaim 6 wherein the portion of the cladding to provide spacing betweenelements consists of two substantially equal parts at opposite sides ofsaid element and defined therebetween a channel to allow cooling liquidto flow past the entire face of said disc elements.
 8. The device ofclaim 7 wherein alternate ones of said lasable elements have claddingremoved from a portion of the circumference of said cladding and theremoved portion is substantially 180* from adjacent elements so thatcoolant liquid flow is in a series arrangement past faces of saidlasable elements.